The capability to calculate the distribution of absorbed dose produced by photon beams and electron beams of the most general characteristics is of vital importance in radiotherapy. Conceptually, this new radiation field model takes as a basis the empirical distributions along three mutually perpendicular reference lines in a "master field" and mathematical expressions to describe the effect of variations of field size, depth and focal distance. This concept is applied to the beam-modifying devices as well. The approach is attractive from a theoretical as well as a practical point of view. The investigations include the generalization for irregular fields modified by irregular blocks for photon beams and electron beams, and the influence of inhomogeneities. The investigations of the problems posed by irregularly shaped and blocked external beams have been completed for photon beams. The extension to electron beams is continuing. Of special interest are the implications of the large number of electron energies and the need for flexible application of different energies and field shapes in combination with photon fields. The central ray distributions can now be described on the basis of only seven characteristic depth dose data points, for 6OCo to 18 MV x-rays, using the concept of Net Fractional Depth Dose (NFD). The NFD formalism is currently being extended to the description of the influence of inhomogeneities, such as lung tissues.